I just realized that i initially made a mistake in my note (probably because i'm a little tired). The correction is this: this is not a 4 vertical shots stitch, this is a stitch from 4 shots taken on the vertical (so 4 horizontal shots one on top of the other).
Having said that, i can understand very well what you mean and i understand perfectly why it appeared. The fact is that i metered on the first shot only and kept the same metering (so that the stitch would work) all the way to the end. Needless to say when you see the outcome, the metered shot is the lowest one. I used a polarizer to saturate the colors nicely in the lower part and indeed it darkened the sky a little too much. So, my friend, i believe you are right: in this case the effect of the filter is a bit too harsh on the reality.
I really do like and appreciate a honest and motivated critique...that's the only way one can learn. So please, as i said in my intro, i prefer a constructive word always!
However, i do think that a polarizer is a great filter and good for all sorts of cases. Surely, it can be used to eliminate undesired reflections and it works superbly, but you can also use it nicely to give back those reflections and exploit that effect. Also, in some landscapes, i think it's a blessing. It can saturate colors very well, it can give some contrast and it can enhance the sky (which can sometimes be too bright if you have a darker FG).
Overall, i like my polarizer and still think it's a necessity for landscapes, too, but (as i said before) in this case you are right - it's too unnatural.
Again, many thanks for your very elaborate comment and also for your time.
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